This invention relates to a direct-current converter, that is, a buck converter, with a controllable switch, with a device for controlling the controllable switch, and with a device for sensing its output current.
A buck converter, that is, a d.c. voltage down converter, is known from DE 196 12 365. The buck converter oscillates at a switching frequency predetermined by an oscillator. To maintain the converter""s output voltage largely independently of the load, the output voltage is supplied to a control device controlling an electronic switch with a correspondingly adjusted pulse duty factor. In order to avoid that the current flowing through the coil exceeds a predetermined value, the control device additionally senses, by means of a sense resistor, the current flowing through the coil during the conducting period of the electronic switch, the current being interrupted by the electronic switch if necessary. This also prevents the output voltage from adopting an undesirably high value.
From DE 33 10 678 there is likewise known a d.c. voltage down converter which differs from the buck converter known from DE 196 12 365 referred to in the foregoing only in that it has no oscillator predetermining a switching frequency but is of the self-oscillating type: The electronic switch will be held conducting until the current flowing through the coil exceeds a limit value, and it will be held non-conducting until the output voltage has dropped below a specified value. From WO 98/24170 a self-oscillating buck converter is known whose oscillatory characteristics are determined only by its output voltage and which includes the added provision of a device with a sense resistor for limiting the coil current.
All buck converters identified are hence controlled by their output voltage. The current flowing through the coil can only be sensed when the electronic switch is closed, sensing in this connection serving only for current limiting purposes.
From DE 39 21 955 a switching controller is known whose power output is controllable. For this purpose the output voltage and the output current are sensed. However, the output current is only sensed when the electronic switch is open.
From EP 0 752 748 a buck converter for charging batteries is known. It includes an electronic switch and a current sense resistor sensing the current flowing through the coil and causing the electronic switch to be turned on and off when the battery voltage lies below a specified value, such that a time-averaged constant charging current results. When the battery is then recharged to a level at which its voltage exceeds the specified value, the electronic switch will be controlled in dependence upon the output voltage of the buck converter, such that the output voltage will be maintained constant.
From WO 99/13559 a self-oscillating buck converter is known whose output current is sensed by a current sense resistor and maintained at a constant level. However, with the input voltage rising, the output current rises correspondingly.
It is an object of the present invention to provide a buck converter of straightforward construction which is designed specifically for charging a storage battery, that is, which supplies a current adapted to the particular state of charge of the storage battery and the prevailing ambient conditions (temperature) irrespective of the input voltage prevailing at the particular instant. It is to be noted that a nearly fully charged storage battery is capable of taking up a small amount of current only whereas a nearly depleted storage battery can take up a comparatively large amount of current.
This object is accomplished in that the buck converter of the present invention includes a device which adjusts the output current level as a function of the respective output and input voltage. Considering that when the buck converter of the present invention is used for charging a storage battery, the output voltage of the buck converter is predetermined by the connected storage battery, reflecting the particular state of charge of the storage battery, the buck converter thus invariably supplies a current adapted to the particular state of charge of the storage battery.
A buck converter of the present invention includes a controllable switch, a device for controlling the controllable switch, a device for sensing the output voltage of the buck converter, and a device for sensing the output current of the buck converter. Through its input terminal the buck converter can be coupled to a power supply, for example. In a manner known in the art the buck converter operates to reduce the input voltage residing at its input terminal to a lower output voltage. Preferably a storage battery is connected to its output terminal. Because the battery voltage varies only very slowly during charging of the battery, the mode of operation of the buck converter of the present invention is first described for the case of a constant output voltage determined by the battery voltage.
The controllable switch, preferably an electronic switch, in particular a transistor, is controlled by the control device in a manner known in the art, that is, it is opened and closed in successive cycles. With the switch closed, that is, during the ON period of the transistor, a current flows from the input terminal through the switch and the coil to the storage battery, causing magnetic energy to be stored in the coil. With the switch open, that is, during the OFF period of the transistor, the magnetic energy is converted into electrical energy, causing a current to flow from the coil to the storage battery, in which process the circuit is closed via a diode in a manner known in the art. The device for sensing the output current of the buck converter of the present invention, preferably a current sense resistor, senses the output current of the buck converter during both the ON period and the OFF period of the electronic switch. The control device of the buck converter of the present invention will close the switch when the output current has dropped to a specified first value, and it will open the switch when the output current has risen to a specified second value. Because hence the buck converter of the present invention is controlled by its output current, this output current is independent of the magnitude of the input voltage. The buck converter of the present invention is, however, designed so that with the output voltage of the buck converter increasing, the first and the second specified value will change such that the switch remains open for a longer period than before, so that the time-averaged output current of the buck converter will decrease.
In a preferred embodiment the buck converter of the present invention is further configured in such fashion that also in the presence of a higher input voltage of the buck converter the first and the second specified value will change so that the switch remains open for a longer period than before. In this manner the so-called storage delay time of the electronic switch is compensated for, which in particular at large voltages needing to be switched causes a delayed opening of the switch and hence a larger output current than would be the case with an ideal buck converter. Therefore, operating the buck converter of the present invention at different input voltages poses no problems, since this does not affect the magnitude of its output current.
The buck converter of the present invention is preferably of the self-oscillating type, that is, it has no oscillator for controlling the opening and closing of the controllable switch. However, the buck converter of the present invention does possess a control input via which the buck converter can be switched off. This can be accomplished, for example, by means of a charge control device issuing a corresponding signal when a storage battery coupled to the buck converter has reached its fully charged condition.
In a buck converter affording particularly simple and hence economical construction, the invention makes provision for the control device to include a first comparator with hysteresis and a first reference voltage source. The hysteresis of the first comparator is produced in that the first reference voltage source supplies two different reference voltages, depending on whether the output of the first comparator is xe2x80x9clowxe2x80x9d or xe2x80x9chighxe2x80x9d.
An advantageous buck converter further includes an arrangement for turning the buck converter off when the input voltage is too low to ensure proper operation of the buck converter, that is, when the voltage at the input terminals drops below a minimum input voltage, the buck converter will turn itself off automatically.
A buck converter which is particularly advantageous for charging a storage battery is configured according to the present invention so that its output resistance value is practically infinite at a low input voltage. This may be accomplished, for example, by opening at least one controllable switch disconnecting the output terminals from the buck converter circuit or preventing current flow between the output terminals. In this manner it is ensured that a storage battery coupled to the output terminals of the buck converter cannot be discharged through the buck converter when the input terminals of the buck converter are (accidentally) shortcircuited or when a voltage lower than the battery voltage resides at these terminals.
The present invention will be explained in the following with reference to several embodiments illustrated in the accompanying drawings. Further embodiments of buck converters of the present invention are dealt with in the description.